Lecture on Amelogenesis

Overview

• Amelogenesis: Formation of enamel by ameloblasts.
• Enamel: Mineralized tissue with 96% inorganic and 4% organic matter.
• Two stages of amelogenesis:
  • Organic matrix formation
  • Mineralization

Stages of Amelogenesis

1. Formative Stage

• Organic Matrix Formation

  • Occurs during the formative stage.
  • Enamel organ enters Advanced Bell Stage.
  • Odontoblasts deposit dentin and signal ameloblasts to begin amelogenesis.
  • Ameloblasts:
    • Tall columnar with Tomes' process (pyramidal process).
    • Deposit organic matter (enamel proteins: 90% amelogenins and 10% non-amelogenins such as ameloblastin and enamelin).

• Mineralization Phase One

  • Immediate partial mineralization of 25-30%.
  • Involves secretion of hydroxyapatite crystals:
    • From Tomes' process:
      • Distal part: parallel to enamel rods.
      • Proximal part: 65 degrees to enamel rods (inter-rod formation).
  • Each ameloblast contributes to 4 enamel rods and vice versa.

2. Maturative Stage

• Mineralization Phase Two
  • Completion of mineralization (70-75% additional mineralization).
  • Ameloblasts lose Tomes' process and undergo modulation (ruffled and smooth surfaces).
  • Enzyme Activity:
    • Release of minerals to increase enamel mineralization to 96%.
    • Enzymes like enamelysin and kallikrein-4 degrade enamel proteins for reabsorption.
  • Mineralization proceeds from dentino-enamel junction to outer surface.

Unique Features of Amelogenesis

• Cell Origin: Enamel is formed by epithelial cells, unlike other mineralized tissues formed by connective tissue origin.
• Ameloblast Fate: Destroyed after enamel formation, disallowing repair or renewal.
• Protein Composition: No collagen in enamel; instead, enamel proteins.
• Mineralization: Immediate partial mineralization (30%) occurs at the outset, unlike other tissues.
• Reabsorption: Enamel proteins are reabsorbed, unique among mineralized tissues.

Summary

Amelogenesis is a unique process of enamel formation, distinguished by its cellular origin, protein composition, and mineralization process. Understanding the dual-phase mineralization and the role of ameloblasts provides insight into why enamel cannot regenerate once fully formed.